Some communications channels or interfaces utilize what is termed “open drain” transmitter circuits, such as those utilized in the interface standard know as a high-definition multimedia interface (HDMI). In the HDMI standard, a receiver circuit provides a current through termination resistors contained in the receiver circuit, and this current then flows through data lines of a communications channel to the open drains of metal oxide semiconductor (MOS) transistors contained in the transmitter circuit. The open drain transistors are turned ON and OFF to thereby communicate data from the transmitter over the data lines to the receiver, with the receiver supplying the power for this communication. The communications channel typically includes a return ground path line through which the current from the receiver is fed back from the transmitter to the receiver.
This conventional approach results in the current that is supplied by the receiver being “sunk” to ground via the ground path line and thus results in this current being used for communicating data over the communications channel but not being otherwise utilized and wasted through sinking the current to ground. Recent approaches have attempted to utilize or “harvest” this current supplied by the receiver to power components contained in the transmitter. Using power harvested from the current provided by the receiver eliminates the need for having an additional power source in the transmitter, assuming this current can supply sufficient power for all components in the transmitter. There are issues, however, in attempting to harvest the current provided by the receiver. In some instances, the resulting voltage that can be generated using the harvested current is insufficient to power all the components in the transmitter (e.g., some transmitter components require a higher voltage than can be generated simply using the harvested current). In other situations, the receiver may be powered by a supply voltage having a sufficiently large value that this receiver supply voltage can damage low voltage components in the transmitter. This is particularly true where the transmitter includes low voltage thin oxide MOS devices, for example.
There is a need for improved circuits, systems, and methods of harvesting power in communications channels utilizing open drain, or open collector, topologies.